Aerospace sealants must satisfy demanding mechanical, chemical, and environmental requirements. Aerospace sealants can be applied to a variety of surfaces including metal surfaces, primer coatings, intermediate coatings, finished coatings, and aged coatings. Sealants comprising sulfur-containing prepolymers that exhibit acceptable fuel resistance, thermal resistance, and flexibility for aerospace applications are described, for example, in U.S. Pat. No. 6,172,179. In sealants such as those described in U.S. Application Publication Nos. 2006/0270796, 2007/0287810, and 2009/0326167, a sulfur-containing prepolymer such as a thiol-terminated polythioether prepolymer can be reacted with a polyepoxide curing agent in the presence of an amine catalyst to provide a cured product. These systems are useful as sealants and can meet the demanding performance requirements of the aerospace industry.
To impart corrosion resistance, a chromate such as hexavalent chromium (chromium (VI)) compounds or strontium chromate (SrCrO4) can be added to a sealant composition. Although chromates can provide excellent corrosion resistance to aluminum surfaces, the heavy metals are toxic. It is therefore highly desirable to develop alternative non-chromate corrosion inhibitors for use in aerospace sealant applications.
Corrosion inhibiting sealants should provide the following functions: (1) rapid polymer exit and short term metal passivation; (2) passivation by metal surface adsorption and double layer space charge altering; (3) forming a water insoluble passivation layer which persists or remains insoluble in neutral, alkaline, and acidic environments; (4) passivation by control of pH or neutralization of acid at the metal/electrolyte interface; and (5) transport out of the polymeric matrix onto adjacent exposed bare metal areas in a moist but no liquid immersed environment.
Examples of non-chromate corrosion inhibitors are disclosed in U.S. Pat. No. 5,951,747 and in U.S. Pat. No. 6,059,867, each of which is incorporated by reference in its entirety.
U.S. Pat. No. 5,917,747 discloses non-chromate corrosion inhibitors for epoxy/polythioether-based sealant compositions comprising at least one inhibitor selected from a phosphate, a phosphosilicate, and a silicate, and at least one inhibitor selected from a titanate and a zinc salt. More particularly, a non-chromate corrosion inhibitor can comprise a borate such as boric acid, a sulfur-containing succinate such as (2-benzothiazolylthio)succinic acid or an amine salt thereof, a phosphate such as calcium dihydrogen phosphate, a phosphosilicate such as calcium, strontium zinc phosphosilicate, a titanate such as sodium titanium oxide, and a zinc salt such as zinc phosphate and/or zinc cyanamide U.S. Pat. No. 5,917,747 does not disclose any properties of a cured sealant important to aerospace sealant applications such as adhesion, tensile strength, elongation, and fuel resistance.
U.S. Pat. No. 6,059,867 discloses non-chromate corrosion inhibitors comprising at least one compound selected from boric acid and boric acid salts, dipotassium monohydrogen phosphate, calcium dihydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate, sodium pyrophosphate, calcium strontium phosphosilicate, calcium, strontium zinc phosphosilicate, and tetra sodium orthosilicate; and (b) at least one compound selected from sodium titanium oxide, an inorganic zinc compound such as zinc borate, and an organo-zinc compound. Specific inhibitor combinations evaluated include one or more of boric acid, calcium dihydrogen phosphate, dipotassium monohydrogen phosphate, sodium pyrophosphate, calcium, strontium phosphosilicate or calcium strontium zinc phosphosilicate, tetrasodium orthosilicate, (2-benzothiazolylthio)succinic acid, sodium titanium oxide, zinc phosphate, and zinc cyanamide. The formulations were only evaluated for galvanic current, electrochemical impedance, filiform testing, salt spray testing, and pH range immersion testing.
However, non-chromate corrosion inhibitor packages for use with polyepoxide-cured thiol-terminated polythioether sealants have not been developed or demonstrated to be effective as aerospace sealants.